1. Field of the Invention
The present invention relates to a fixing device for use in image forming apparatus such as copying apparatus, laser printers and the like.
2. Description of the Related Art
Conventionally, fixing devices for image forming apparatus are generally known and have the capacity to fuse and fix a toner image as a copy sheet bearing an toner image in an unfixed state passes between a pair of pressure and heating rollers. Control circuits for maintaining the surface temperatures of the aforesaid pair of fixing rollers at predetermined temperatures generally compare the temperature detection signals emitted from temperature detection sensors such as thermistors and the like disposed so as to be in contact with the roller surface with predetermined reference temperature signals used for temperature regulation. The surface temperature of the roller is controlled by turning on an electrical current which is supplied to a heater provided within the roller when the temperature detection signal is less than the reference temperature signal, and by turning off the electrical current flowing to the heater within the roller when the temperature detection signal is greater than the reference temperature signal.
In the aforesaid arrangement, however, a certain amount of time elapses while the heat emitted from the heater within the roller is transmitted to the surface of the roller and, therefore, some time is required for the thermistor to detect the surface temperature and generate a output corresponding to the detected temperature. Accordingly, in the previously described control method, even if the heater is turned on or off, some time delay is produced until the temperature effect of the heater becomes the actual temperature detection signal to be detected, and the time delay causes greater fluctuation in the roller surface temperature in regulating the temperature.